14.3 The RS-232 Serial Protocol

One of the most common serial interfaces is based on the RS-232 standard, which was developed primarily to allow individuals to use terminals with remote computer systems over a telephone line more easily.

The basic configuration of a terminal and a computer connected by two modems is shown in Figure 7.2 .

Figure 14.2: A terminal and a computer connected by two modems

The computer and terminal are called data terminal equipment ( DTE ), while the modems are called data communication equipment ( DCE ). The standard RS-232 connector is a 25-pin D-shell type connector; only nine pins are used to connect the DTE and DCE sides together, as seen in Figure 14.3 .

Figure 14.3: The standard 25-pin RS-232 connector

Of these nine pins, only transmit data (pin 2), receive data (pin 3), and signal ground (pin 7) are essential for directly wired communications. Five pins (2, 3, 7, 8, and 20) are needed for proper operation of modems. Frame ground (pin 1) was originally used to connect electrically the physical frame of the DCE and the frame of the DTE to reduce electrical hazards, although its use has been discontinued in the RS-232-C standard. Table 14.1 describes the use of each pin.

Table 14.1: .RS-232 Pin Assignments for a 25-Pin Connector

Pin	Code	Name	Description
1	FG	Frame Ground	Chassis ground of equipment. (Note: this pin is historical; modern systems don't connect the electrical ground of different components together, because such a connection causes more problems than it solves.)
2	TD	Transmit Data	Data transmitted from the computer or terminal to the modem.
3	RD	Receive Data	Data transmitted from the modem to the computer.
4	RTS	Request to Send	Tells the modem when it can transmit data. Sometimes the computer is busy and needs to have the modem wait before the next character is transmitted. Used for "hardware flow control."
5	CTS	Clear to Send	Tells the computer when it's okay to transmit data. Sometimes the modem is busy and needs to have the computer wait before the next character is transmitted. Used for "hardware flow control."
6	DSR	Data Set Ready	Tells the computer that the modem is turned on. The computer should not send the modem commands if this signal is not present.
7	SG	Signal Ground	Reference point for all signal voltages.
8	DCD	Data Carrier Detect	Tells the computer that the modem is connected by telephone with another modem. UNIX may use this signal to tell it when to display a login: banner.
22	RI	Ring Indicator	Tells the computer that the telephone is ringing.
20	DTR	Data Terminal Ready	Tells the modem that the computer is turned on and ready to accept connections. The modem should not answer the telephone - and it should automatically hang up on an established conversation - if this signal is not present.

Because only eight pins of the 25-pin RS-232 connector are used, the computer industry has largely moved to smaller connectors that have fewer pins and cost less money to produce. Most PCs are equipped with a 9-pin RS-232 connector, which has pin assignments: listed in Table 14.2 .

Table 14.2: .RS-232 Pin Assignments for 25- and 9-pin Connectors

Code	Name	25-pin Connector Assignment	9-pin Connector Assignment
TD	Transmit Data	2	3
RD	Receive Data	3	2
RTS	Request to Send	4	7
CTS	Clear to Send	5	8
DSR	Data Set Ready	6	6
SG	Signal Ground	7	5
DCD	Data Carrier Detect	8	1
DTR	Data Terminal Ready	20	4
RI	Ring indicator	22	9

A number of nonstandard RS-232 connectors are also in use. The Apple Macintosh computer uses a circular 9-pin DIN connector, and there are several popular (and incompatible) systems for using standard modular telephone cords.

In general, you should be suspicious of any RS-232 system that claims to carry all eight signals between the data set and the data terminal without the full complement of wires.